It's possible to execute a roll with the acceleration vector
continuously pointing in the proper direction and with continuous
positive G forces, but it is not possible to do it with a constant
level of acceleration. That is, you can keep the acceleration
positive, but you cannot hold it at 1.0 Gs. As long as it is positive
and the vector is stationary (both of which are possible), passengers
will have only the sensation of slightly rising or falling in a
straight line during the roll.

I've seen Bob Hoover demonstrate this by pouring fruit juice into a
cup during a barrel roll. Others have probably done the same thing.

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